The British-Indian director Gurinder Chadha wasn’t yet born when the supposedly peaceful transfer of power from Britain to India took place in 1947, but the ensuing horrors of Partition engulfed her Sikh relatives and swept them up in devastation. One of her aunts, a mere child, starved to death. The division of the country between religious factions is now known to have brought about the largest mass migration in human history, during which 14 million Hindus, Sikhs and Muslims were displaced.

Viceroy's House trailer

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How to dramatise being caught in the midst of all this? Best known for crowd-pleasing comedies such as the genuinely sparky Bend it Like Beckham (2001), Chadha has been absent from our screens since her 2010 neo-Ealing farce It’s a Wonderful Afterlife, a garish fiasco over which at least seven veils should be swiftly drawn. She has upped her game massively for Viceroy’s House, which revisits the build-up to Partition and engages in a serious-minded way with the mess it made of so many lives. So, credit where it’s due: this is easily Chadha’s most ambitious film to date, and her best in a long time.

The access point for her British audience is Downton-esque – it’s the upstairs-downstairs intrigue in the grand New Delhi residence that housed India’s last viceroys, up to the final man given the post by George VI: this is Lord “Dickie” Mountbatten, in March 1947.

Hugh Bonneville plays this glamorous minor royal, fresh off the plane as the film is starting, with a dashing variation on his usual stiff upper lip, and a determination to keep India united under his watch. The script accentuates how out of his depth Mountbatten would become in the negotiations that followed; in fact, it goes so far as to posit him, perhaps controversially, as a stooge and scapegoat for a secret Partition plan his taskmasters had been preparing for many years before he landed.

As Edwina Mountbatten, Gillian Anderson has come up with a curious, stiff walk to match her necessarily strangulated accent, and builds a good sense of how much practical energy this Vicereine devoted to the crisis. She gets stuck in as an aid worker at the end, but even at the start, she has a scene sacking an intolerant housekeeper who recoils from the Indian staff (500 of them!) at her beck and call.

While sometimes a fraction too squawky, it’s an intelligent and lively take on a tricky role. Edwina’s notorious promiscuity, though, is nowhere to be seen – you’d have to wait for another film, say, to fill you in on her intense friendship with Jawaharlal Nehru (excellent Tanveer Ghani), the leader of the independence movement, who spends the few scenes he has combatting Muhammad Ali Jinnah (Denzil Smith), his separatist Muslim rival.

There’s only so much in this desperately involved historical saga that Chadha and her screenwriters are able to grapple with, of course: supporting roles for Michael Gambon, as Dickie’s wily chief of staff, and Simon Callow, as the boundary commissioner, give both of these front-of-house players some nice, chewy moments. But the focus isn’t always judicious.

Viceroy's House: Breakfast on the Lawn

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The reason for a relatively sketchy take on the Mountbattens, and indeed all the Nehru-Jinnah-Gandhi horse-trading, is to save space for a Romeo-and-Juliet-ish love story between Jeet (Manish Dayal), one of Dickie’s new Hindu servants, and his hoped-for Muslim bride Aalia (Huma Qureishi). Intended as the main event, with AR Rahman’s score as a matchmaking device, this aspect of the film is too cute for the context: while thoroughly easy on the eye, the couple are just generically star-crossed. What little they have say for themelves, they have to say in English, worse luck.

Chadha has saved a big chunk of her budget for the very end, which must convey the chaos, scale and bloodshed of this mismanaged upheaval while knitting the story’s threads together. Finally the Jeet-Alia story pays off, because Chadha channels her inner David Lean with a full-blooded, matinee shamelessness that takes you aback. With a couple of sweeping, hankies-at-the-ready gestures, she wrests all thought of Partition away from its bickering white architects, and honours the nameless casualties instead.